The directional coupler is an arrangement related to the transmission path of a radio-frequency electromagnetic field. It gives a measurement signal the level of which is proportional to the strength of a field propagating to a particular direction in the transmission path. In principle, a field propagating to the opposite direction in the transmission path does not affect the level of the measurement signal. The directional coupler has at least three ports: an input, an output and a measurement port. The energy of a signal incoming to the input port is led almost totally through the coupler to the output port, and a small part of this energy is transferred to the measurement port. The part of the directional coupler between the input and output ports is at the same time a part of the transmission path of a radio apparatus which continues, for example, to the antenna of a transmitter. Then, a measurement signal proportional to the actual strength of the field propagating towards the antenna is received from the measurement port, which signal can be used in the controlling purposes of the transmitter. The accuracy of the control is partly dependent on the quality of the directional coupler, that is, of how completely the effect of the field propagating in the opposite direction in relation to the field to be measured is eliminated.
In this description and claims, the “forward signal/field” means a signal/field propagating from the input port to the output port of the directional coupler and the “reverse signal/field” means the signal/field propagating from the output port to the input port of the directional coupler.
A directional coupler may be designed in several ways. Most of them are based on the utilisation of transmission lines of quarter-wave length. FIG. 1 shows an example of such known directional coupler. In it, the transmission path of the signal to be measured comprises the transmission conductor 110 which is a first conductor strip on the upper surface of a circuit board PCB, and the signal ground GND which consists of the conducting lower surface of the circuit board. The head end of the first conductor strip 110 together with a conductor pad connected to the signal ground constitute the input port P1 of the directional coupler. Correspondingly, the tail end of the first conductor strip together with the signal ground constitute the output port P2 of the directional coupler. Additionally, on the upper surface of the circuit board PCB, there is a second conductor strip 120 parallel to the first conductor strip, the length of which second conductor strip is a quarter of wavelength λ at the operating frequencies of the directional coupler. The distance between the conductor strips 110 and 120 is for example a tenth of their distance from the ground. The second conductor strip 120 continues at its ends away from the first conductor strip. The first extension 121 ends at the third port, or the measurement port P3. When the directional coupler is in use, a circuit has been coupled to the measurement port the impedance Z of which circuit is equal to the characteristic impedance Z0 of the transmission lines formed by the conductor strips of the directional coupler together with the signal ground and the medium. The second extension 122 of the second conductor strip ends at the fourth port P4 which is also called the isolated port here. Thus the directional coupler of the example has four ports, as do also most other directional couplers.
The second conductor strip 120 acts as a sensing conductor: Because of the electromagnetic coupling between it and the first conductor strip, part of the energy fed to the input port transfers to the circuit of the second conductor strip, to the load impedances of the ports P3 and P4. When the frequency of the forward field is such that the λ/4 condition aforementioned and drawn in FIG. 1 is fulfilled, the energy transferring to the measurement port P3 is at its maximum, and the energy transferring to the isolated port P4 at its minimum. The latter energy is zero in an ideal coupler, because even and odd waveforms occurring in the coupler cancel out each other in the isolated-port end of the transmission line based on the second conductor strip 120. The directivity of the coupler is based on this fact. Namely, if a reverse field of equal frequency exists in the directional coupler, almost none of its energy is transferred to the measurement port P3 because of the symmetrical structure. The quality of directivity is expressed as the proportion of the signal level in the measurement port to the signal level of the isolated port. This is the same thing as the relation of the signal level caused by the forward field to the measurement port to the signal level caused by the reverse field in the measurement port, when these fields propagating to opposite directions are of equal frequency and strength.
FIG. 2 shows an example of the directivity and bandwidth of the directional coupler according to FIG. 1. The figure shows the curves of two transmission coefficients as the function of frequency. Curve 201 shows the variation of the signal level in the measurement port in proportion to the level of the input signal, and curve 202 shows the variation of the signal level in the isolated port in proportion to the level of the input signal. The difference of coefficients expressed in decibels indicates the value of directivity. It appears from the curves that the directivity is at its highest about 20 dB which value is only valid in a frequency range the relative width of which is only a few percentages on both sides of the frequency 2.08 GHz corresponding the quarter wave. Directivity exceeds the value of 10 dB in the range of 1.8-2.45 GHz, the relative width of which is about 30%. Curve 201 also indicates that, in the operating range of the directional coupler, the signal level in the measurement port is about 25 dB lower than the signal level passing through the coupler. This means that the coupler causes a 0.014 dB attenuation to the passing signal.
If the directional coupler is used at a frequency in which the length of the parallel parts of conductor strips 110 and 120 corresponds a half wavelength, the situation in the third and the fourth port is reversed: the energy transferring to the third port P3 is at its minimum, and the energy transferring to the fourth port P4 is at its maximum. If the directional coupler then is used at frequencies which are low compared to the frequency corresponding the length of the quarter wave, directivity is very low.
The aforementioned value of directivity, 20 dB, typical in directional couplers according to FIG. 1, is still unsatisfactory. This relatively modest value is caused by the even and the odd waveform not totally cancelling out each other on the side of the isolated port, because the odd waveform also propagates in addition to dielectric medium at a greater amount in air, in which case its velocity is greater. A better structure by its directivity is achieved if both the conductor of the transmission path and the sensing conductor are arranged inside a dielectric board on both sides of which there is a ground plane. Directivity will also be improved when using a totally air-insulated transmission line. However, a further disadvantage of all directional couplers using lines of λ/4 length is that they function satisfactorily only in a relatively narrow frequency range and that they require a relatively large space.